1. Field of the Invention
The present invention relates to the ink jet head of an ink jet printer. More particularly, the invention relates to an ink jet head having a number of ink nozzle arrays arranged in the main scan direction, each of which is provided with a number of ink nozzles arranged in the sub-scan direction.
2. Related Background Art
In recent years, an ink jet printer has been in use generally as a printing apparatus. It is required for such printing apparatus to form images in high quality at high speed. The ink jet printers generally in use form dot-matrix images on a printing sheet by ink droplets discharged from the ink jet head in such a manner that while the ink jet head travels in the main scan direction, the printing sheet moves in the sub-scan direction.
For the generally used ink jet head, many numbers of ink jet nozzles are arranged in the sub-scan direction for the nozzle arrays, and for the full-color use ink jet head, the nozzle arrays are arranged for a primary color or three primary colors in the main scan direction. Also, among ink jet printers, there is the one in which the ink jet head driven to travel in the main scan direction is made to travel in both the forward and backward directions for the high-speed image formation.
For example, the ink jet printer disclosed in the specification of Japanese Patent Application Laid-Open No. 2001-171119 is arranged to provide the ink jet head thereof with two columns of nozzle arrays each for use of the three primary colors, YMC. Such nozzle arrays for the YMC use are arranged to be symmetrical in the main scan direction.
In other words, six columns of nozzle arrays are formed in the order of a first C use, a first M use, and a first Y use, and a second Y use, a second M use, and a second C. Then, for the first YMC uses, and the second YMC uses, ink nozzles are arranged in the same cycle, but the phases thereof are arranged to be reciprocal just by a portion equivalent to a half cycle.
Then, the ink jet printer disclosed in the specification of the aforesaid Japanese Patent Application, for example, operates the nozzle arrays of the first and second YMC use both in the reciprocal traveling of the ink jet head so as to print high resolution images at high speed. Here, the first and second nozzle arrays of the ink jet head for the YMC use are arranged in the same cycle but in the phases which are reversal just by a portion equivalent to a half cycle. Therefore, the arrangement density of the main-scanning columns of YMC colors of a printed image in the sub-scan direction is made twice as much of the arrangement density of the ink nozzles of each nozzle array. Consequently, the printed image thereof is in high resolution.
In this respect, even the pixel, on which ink droplets of YMC colors are impacted at the same position on a printing sheet, may result in different coloring depending on the impact order of ink droplets, “YMC” or “CMY”. However, in accordance with the ink jet printer disclosed in the specification of the aforesaid Japanese Patent Application, the first and second YMC use are arranged symmetrically in the main scan direction for the ink jet head that forms images both in the reciprocal traveling to make it possible to from the pixel having the impact order of “YMC” and the pixel having the impact order of “CMY” both in the reciprocal traveling of the ink jet head. The resultant coloring of the printed image is excellent.
Also, in the specification of the aforesaid Japanese Patent Application, it is also disclosed that only the first nozzle array for YMC use can be operated in the forward movement of the ink jet head, and only the second nozzle array is operated in the backward movement so that an image of low resolution can be formed at high speed with only the pixels having the same impact order.
The ink jet head disclosed in the specification of the aforesaid Japanese Patent Application is capable of forming high-resolution color images at high speed in a good coloring condition. However, in recent years, it has been required to provide images in a quality still higher. In order to enhance the image quality in the general printing, it should be good enough if only the diameter of each ink nozzle is made smaller, while the ink nozzles are arranged in higher density. For the ink jet head, a driving element is incorporated in each of the ink nozzles, which is wired to a driving circuit. Therefore, the enhancement of the arrangement density depends on the manufacturing technologies and techniques thereof.
Here, with respect to the formation method of color images by use of an ink jet printer, the pseudo-formation of the secondary colors are executed by changing the impact density of ink droplets of YMC colors on a printing sheet. As a result, the pixel density of the secondary colors becomes far larger than the impact density of the ink droplets of YMC colors eventually. For example, if should it be possible to adjust the liquid amount of ink droplet freely per ink nozzle, the pixel density of the secondary colors can be made equal to the impact density of the ink droplet. However, it is extremely difficult to arrange this with a generally used ink jet head.
Here, the problem of the arrangement density described above can be solved in such a way that the nozzles for use of large liquid droplets and the nozzles for use of small liquid droplets are arranged individually to be able to discharge ink liquid droplets in the direction perpendicular to the heater board, which is the substrate having heat generating resistive elements formed thereof for use of discharging ink.
For the aforesaid mode, in which ink droplets are discharged in the direction perpendicular to the heater board having the heat generating resistive elements formed for use of ink discharge use, there is a need for the installation of a plurality of ink supply ports for supplying liquid of each color to one heater board when discharge ports for use of a plurality of colors are provided for one heater board. Further, in order to attain the high-speed printing, it is required to increase the number of heaters on one heater board, and the number of discharge ports arranged for each of the heaters as well.
Moreover, the size of the heater board tends to be made larger with the increased numbers of heaters and ink supply ports on one heater board. Nevertheless, in order to manufacture a recording head at cost of as lower as possible, it is necessary to downside the heater board as much as possible. As a result, there is a need for making the areas other than the one occupied by the heaters on the heater board as small as possible.
Now, generally, for an ink jet head, the discharge element that discharges ink is incorporated per ink nozzle, and also, the driving circuit and others are incorporated for driving the discharge element. When these element and driving circuits are driven, heat is generated unavoidably, because these members are actuated by means of electric power.
In this respect, the causes of heat generation of the ink jet head described above are heat generated by the discharge element that discharge ink per ink nozzle; heat generated by the driving circuit that drives the discharge element; and heat generated by wiring that connects the driving circuit and the discharge element, among some others. However, when ink is heated by the discharge element to bubble for effectuating discharge from the heat-generating element, the heat generation is particularly conspicuous by the heat-generating element. At the same time, cooling is also conspicuous by the discharge of the ink droplet thus heated.
Further, the ink jet head that performs discharges by bubbling ink by means of heating given by the heat-generating element is caused to change the temperature of ink retained inside thereof when the temperature of the head changes. As a result, the timing of bubbling and discharging is caused to fluctuate. Consequently, for example, if the temperature of the ink jet head changes significantly at a position in the main scan direction, the timing of ink droplet discharges by the plural nozzle arrays thus arranged is not synchronized, leading to the degradation of the quality of images to be formed.
On the other hand, when a plurality of ink supply ports are arranged in parallel on the substrate for use of a plurality of colors as described above, the ink supply ports themselves provide function to insulate the thermal conduction to the inside of the head. This may present a cause that inevitably generate the varied head temperatures between each of the ink supply ports depending on the nozzle array structure on the portion laying between the ink supply ports inside the head.